DE102011051954B4 - Photovoltaic system with bias on the inverter - Google Patents

Abstract

Apparatus for converting an input DC electrical voltage from a photovoltaic generator (2) to an output AC voltage having at least one transformerless inverter (3) having a DC input (7) for connecting a photovoltaic generator (2) and an AC output (8); a bias voltage generator (43) for applying a bias potential to the AC output (8) of the inverter (3), thereby also affecting the voltage potential at the DC input (7) of the inverter (3); and an inductive RF decoupler (52) adapted for RF decoupling of the AC side from the DC side of the inverter (3), the inductive RF decoupler (52) having an inductance (53) connected in series with the Bias generating means (43) is connected in a branch (42) between ground (44) and a neutral conductor (22, 41) passed from the DC side to the AC side of the inverter (3), or the inductive RF decoupling means (52) is one of the plurality the phase-leading output terminals (12, 13, 14) of the inverter (3) has a corresponding number of inductors (54, 56, 57) connected respectively to an output terminal (12, 13, 14) and to a common connection point (58) with biasing means (43) connected between ground (44) and common connection point (58).

Description

The present invention relates to a device for a photovoltaic system with a bias voltage generating device for generating a bias voltage to an inverter of the photovoltaic system.

Photovoltaic systems with a photovoltaic generator that generates electrical energy from solar energy, an inverter that converts a DC voltage provided by the photovoltaic generator into AC voltage and feeds it, for example, into a power supply network, are finding increasing use. A photovoltaic system may have a plurality of photovoltaic cells and / or inverters. Individual photovoltaic cells are usually connected in series with a module and one or more modules in strings, which form the photovoltaic generator. This then supplies a high DC voltage to over 1000 volts to the inverter, which converts them into an AC voltage and an AC current with an amplitude, phase and frequency conforming to the network to be fed. For this purpose, single or three-phase inverters with and without transformer are known.

DE 102 21 592 A1 describes a transformerless photovoltaic inverter in H or full bridge construction, wherein the connecting lines for tapping the bridge voltage are each connected via a choke coil with the AC output terminals. The choke coils are used to smooth the alternating current generated by the inverter on the output side. With symmetrical timing of the switches of the H-bridge circuit of the inverter, and if the inductors used have identical inductances, can be attributed to the high-frequency Takturig the switch attributable potential jumps.

DE 10 2004 037 446 B4 describes a transformerless photovoltaic inverter in a half-bridge construction in the one to three-phase configuration with a DC link at the input of the inverter having two capacitors connected in series, the junction of which is electrically connected to a neutral conductor which is connected to a protective conductor. Between the photovoltaic generator and the intermediate circuit, a symmetrical DC voltage booster is arranged, which converts the generator DC voltage in a symmetrical with respect to the ground potential DC link voltage. At the output of the inverter, LC line filters are provided to smooth the current flow.

Transformerless inverters, which convert power into the mains AC voltage without galvanic isolation of the photovoltaic generator, are preferred because they are smaller in size, less expensive and have a significantly better efficiency compared to transformer-based inverters. However, due to the lack of electrical isolation between the DC and AC side, it is not possible to define a defined potential with respect to the earth, for example by grounding, at the positive or negative pole of the photovoltaic modules. A defined potential compared to the ground potential at the photovoltaic generator but may be required to z. For example, it is necessary to comply with legal requirements regarding upper limits for potential differences at plant components in order to enable the detection of insulation faults in order to avoid TCO corrosion or deterioration of efficiency in certain photovoltaic modules, etc.

For example. It has been found that on certain photovoltaic modules, in particular on thin-film modules, irreparable damage can be formed by so-called "TCO corrosion", corrosion on a transparent, electrically conductive coating, the so-called TCO (Transparent Conductive Oxide) layer. The TCO layer corrodes under the influence of moisture and heat, which can lead to failure of individual areas of photovoltaic modules and ultimately to the loss of performance of the entire module. It is known that the TCO corrosion forms faster or more intensified when the photovoltaic cells have negative electrical potentials to earth.

Conversely, in crystalline photovoltaic modules with back contacting of the photovoltaic cells efficiency losses have been observed if the photovoltaic cells have a positive voltage potential to the ground during operation. The efficiency losses appear to be due to a polarization effect due to a static charge built up on the surface of the photovoltaic cell, and can be reduced if the photovoltaic cells are kept below ground potential, that is, at a more negative potential than ground potential.

For the reasons mentioned above, the manufacturers of such photovoltaic modules recommend using inverters with galvanically isolating transformers, which allow the application of a defined potential with respect to the ground potential at the photovoltaic generator. At the same time, depending on the type of photovoltaic generator used, its negative or positive pole should be earthed. In this way it can be avoided that the photovoltaic cells of thin-film modules or back contacted crystalline modules have a negative or can accept positive tension against the earth. However, this is made possible by the use of a transformer-based inverter, which is heavier and larger, increases manufacturing and installation costs and reduces efficiency.

In the WO 2008/154918 A1 is proposed in connection with thin-film modules, a transformerless inverter unit having a switched between the negative Photovoltaikgeneratorpol and the negative input of the inverter boost converter. By using the boost converter, the characteristic curve of the photovoltaic modules can be influenced in such a way that negative electrical potentials to the earth and thus possible corrosion damage are reduced. However, the boost converter also causes higher manufacturing and operating costs and efficiency losses.

WO 2010/078669 A1 describes a photovoltaic power plant with a photovoltaic generator and a transformerless inverter and proposes to provide at the AC output of the inverter, a potential shifter which superimposes a voltage component on the AC voltage, by which also the potential on the DC input side of the inverter can be raised or lowered. The superposed voltage of the potential shift device can be DC voltage and / or AC voltage. The superimposed voltage can also be regulated by acquiring measured values at the input of the inverter.

Similarly, also in the WO 2010/051812 A1 and in the publication "Ground Voltage Control (GVC) For Transformerless Inverters In PV Power Plants Preventing TCO Corrosion And Polarization Effects" by Dr. Ing. F. Haase, dr. U. Borup, 25th European Solar Photovoltaic Solar Energy Conference and Exhibition, 6-10 September 2010, Valencia, Spain, proposed to provide at the AC output of a transformerless photovoltaic inverter, an offset voltage source, through which the DC potential at the input side of the inverter can be controlled indirectly, to make all potentials at the connections of the photovoltaic modules to earth positive or all negative depending on the type of module. The offset voltage can be controlled, programmed or switched off. By monitoring the current flowing from the offset voltage source, insulation errors can be detected.

The proposed superimposition of a bias voltage (offset voltage, offset voltage) at the AC output of a transformerless inverter can ensure that the potential on the photovoltaic modules never exceeds or falls below a certain voltage value as required. Thus, the aforementioned TCO corrosion and polarization effects can be avoided. However, a transformer is required at the output of the inverter, which transforms the output voltage of the inverter to a desired voltage for transmission over power lines or for feeding into a network. The bias voltage superimposed on the alternating voltage is then insignificant due to the subsequent transformation.

The potential shift device or offset voltage source of the previously known photovoltaic systems is connected between earth and the neutral point or neutral terminal of the output-side transformer. As a result, the potential of the neutral point or neutral point is no longer free, but is held or clamped. However, this clamping of the potential of the neutral point or neutral point can lead to leakage currents. Conventional inverters for photovoltaic systems have full or half bridge switched switching elements which are clocked at a high frequency of up to about 20 kHz according to a given clock pattern and modulation scheme. Due to this high-frequency clocking of the switching elements jumps in a three-phase inverter configuration, the voltage potential at the AC voltage terminals and in particular at the neutral point or neutral point with a relatively high amplitude and high frequency, which corresponds to the triple clock frequency. If now the potential of the neutral point or neutral point is recorded, parasitic capacitances of a photovoltaic generator, which are relatively large, at least much larger than corresponding parasitic capacitances on the output side of the inverter, are reloaded, which high displacement currents to the photovoltaic generator and thus high leakage currents on the DC side of the inverter are connected.

As such, the capacitive leakage currents are reactive currents and thus lossless. However, they may under certain circumstances, if they z. B. drain on the protective conductor, represent a potential hazard, because at a break in the protective conductor and simultaneous contact of the photovoltaic housing a life-threatening body current could flow through the touching person. This should be avoided.

In addition, it is difficult to differentiate between a leakage current and a fault current due to an error, eg. As a faulty insulation, flows in a contact of a live line with a grounded person. To ensure adequate personal protection must be electrical equipment as Precaution to be disconnected from the mains for a given fault current. In this respect, the inverter can autonomously disconnect from the grid here, if the leakage current is too large. This can disturb the operation and reduce the power generation.

In addition, leakage currents, when flowing as circulating currents across the inverter and the bias source, can damage them. In addition, leakage currents make the detection of insulation faults difficult.

On this basis, it is an object of the invention to eliminate the above shortcomings and in particular to propose measures or means that can absorb such leakage currents or substantially reduce. In particular, it is an object of the invention to provide a device for converting a DC electrical voltage into an AC voltage with a transformerless inverter suitable for operation with thin film photovoltaic modules as well as crystalline back contacting modules or other modules requiring a defined maximum or minimum potential , but suitable, but avoiding leakage currents and associated disadvantages or at least largely reduced. This preferably with simple and preferably also retrofittable to existing equipment.

Another object of the present invention is to provide such a device, which also enables a reliable detection of insulation faults, especially creeping insulation faults in operation.

This object is achieved with the device according to claim 1 or the additional module according to claim 14.

According to a first aspect of the invention, a device is provided for converting an input-side direct electrical voltage from a photovoltaic generator into an output-side alternating voltage. The device according to the invention has an inverter which has a DC voltage input for connecting a photovoltaic generator and an AC voltage output. The inverter is transformerless and thus able to convert the input-side DC voltage with high efficiency in the output side AC voltage.

The device according to the invention further comprises a bias voltage generating means for applying a bias potential to the AC output of the inverter, thereby indirectly also indirectly affecting the voltage potential at the DC input of the inverter. For example. allows the bias voltage generating means, by applying a positive or negative bias potential to the AC output of the inverter to raise or lower the voltage potential at the DC input of the inverter such that the potential at the negative pole of a connected photovoltaic generator has a predetermined minimum value, for. B. 0 volts, against protective earth (PE) does not fall below or the positive pole a predetermined maximum value, z. B. 0 volts, against PE does not exceed. As a result, it can be ensured that all photovoltaic cells are kept either above (at more positive potentials) or below (at more negative potentials than) the ground potential, so that harmful TCO corrosion of thin-film modules or polarization effects in the case of back-contacted crystalline modules can be avoided or can. This provides the basis for high efficiency in the conversion of solar energy into electrical energy.

The invention is further characterized by an inductive high frequency (RF) decoupling device arranged and arranged for RF decoupling of the AC side from the DC side of the inverter. According to the invention, the inductive RF decoupling means comprises an inductor connected in series with the biasing means in a branch between ground and a neutral conductor passed from the DC side to the AC side of the inverter. Alternatively, the inductive RF decoupling device has a number of inductances corresponding to the number of phase-carrying output terminals of the inverter, which are each connected to an output terminal and on the other hand to a common connection point, wherein the bias voltage generator is connected between ground and the common connection point. The inductive RF decoupling device provides this high-frequency decoupling by allowing potential jumps on the AC side of the inverter at a constant average value of the potential corresponding to the predetermined value of the bias voltage by the bias voltage generating means. As a result, the inductive RF decoupling device according to the invention prevents a feedback of the potential jumps to the DC side of the inverter and consequently to the photovoltaic generator and associated displacement currents for the transfer of parasitic capacitances of the photovoltaic generator. In other words, blocks or attenuates the inductive RF decoupling device any leakage currents and prevents them from being generated at the photovoltaic generator and as a circulating currents over the Inverter and the bias generator can flow and damage them.

The device according to the invention preferably also has a photovoltaic generator with at least one photovoltaic module in order to form a photovoltaic system. By the bias voltage generating device and thin-film modules or crystalline modules with back contacting as photovoltaic modules can be used gently and effectively.

The inverter according to the invention is preferably arranged in the three-phase configuration with three AC output terminals, each associated with one of the three phases of the output voltage. Of course, the inverter may also include three separate inverter units for the individual phases, and these may be coupled to the same or separate photovoltaic generators.

The inverter may be configured in the full or half bridge configuration of high frequency clocked switches as are well known in the art. The half bridge configuration is preferred because it reduces the number of switch elements and associated costs, switching losses and control overhead.

Preferably, the inverter according to the invention has a balancing circuit which serves to set up the voltage potentials at the input terminals of the inverter substantially symmetrically with respect to the bias potential predetermined by the bias voltage generating means. In a preferred embodiment of the invention, the inverter has for this purpose a DC intermediate circuit with two energy buffers connected in series with one another between the DC voltage input terminals, which are formed, for example, by identically dimensioned capacitors. The connection point between the energy buffers is electrically connected to a neutral conductor, to which the bias voltage generator is also connected. Thus, the predetermined bias potential is on average also at the connection point between the energy buffers. The balun could also be formed by a voltage divider based on resistors, albeit lossy, or by inductors. In any case, it can be achieved by the balancing that the potentials at the positive and negative input terminal of the inverter are symmetrical with respect to the neutral conductor, so that only half a photovoltaic generator voltage as a bias sufficient to the potentials for all PV modules on either a non-negative or non-positive potential to hold in relation to the earth potential. This simplifies any control of the bias of the bias generator.

The photovoltaic system according to the invention preferably further comprises a power transformer connected to the AC output of the inverter, which adjusts the inverter output voltage to grid sizes of a network to be fed. For example, the inverter output voltage is transformed to the AC voltage of a public power grid or to a high voltage suitable for transmission over high voltage power lines. Advantageously, the output transformer allows galvanic isolation between the AC output side of the inverter and the adjoining network. As a result, the bias potential given by the bias voltage generator at the output of the inverter does not affect the mains voltage.

Incidentally, the transformer preferably has a primary side connected to the alternating voltage output of the inverter, a secondary side for connection to the mains and a neutral connection to the primary side, which is preferably connected to a neutral conductor. Particularly preferably, the neutral conductor is also connected to the midpoint between the capacitors or energy stores in the DC intermediate circuit, passed from the DC side to the AC side of the inverter and also connected to the bias voltage generating means. This simplifies the design and control of the inverter and the bias generator.

The power transformer is preferably a three-phase transformer having three outer conductor strings connected to a three-phase inverter or to separate single-phase inverters. In the star connection, the other ends are then connected together at the star point, wherein the star point preferably forms the neutral terminal, which is electrically connected via the neutral conductor to the bias voltage generation device and the connection point between the intermediate circuit capacitors. The biasing device can also be connected to the outer conductor strands of the primary side of the power transformer or the AC output lines of the inverter.

The bias generator has a constant voltage source between ground and the AC output of the inverter is connected. The constant voltage source may be variable so that the magnitude and polarity of the bias voltage provided by the constant voltage source may be controlled in operation.

The inventive inductive RF decoupling device is provided for RF decoupling of the photovoltaic generator from the AC input side of the inverter. In a particularly preferred, easily implementable embodiment, the inductive RF decoupling device is formed by a single inductance, for example a choke, coil or the like, which is connected in series with the bias voltage generating device between earth and the neutral conductor. The neutral conductor is preferably connected to the star point of the network transformer connected to the output of the inverter, as explained above, and preferably also to the protective conductor of the photovoltaic system.

The inductance acts as an HF resistor for the separation or damping of high-frequency leakage currents. It enables the potential jumps at the star point and prevents them from being fed back to the DC side. At the same time, a mean potential corresponding to the bias predetermined by the bias voltage generating means is maintained at the neutral point, which also determines the (average) potential on the DC side of the inverter or on the photovoltaic generator.

In the alternative embodiment of the inductive RF decoupling device according to the invention, this has the number of phases of the inverter output voltage corresponding number of inductors, which are each connected to an AC output terminal of the inverter and on the other hand connected together at a common connection point. The bias voltage generating device is then connected to the connection point. Advantageously, this embodiment of the RF decoupling device according to the invention can be used in mains transformers both in star connection and in delta connection, because the common connection point of the inductors forms a virtual neutral point for the connection of a bias voltage generating device. Incidentally, the plurality of inductors for separating or damping high-frequency leakage currents are effective in the same way as the single inductance of the embodiment explained above.

In a preferred embodiment of the invention, the apparatus further comprises a sensor device for detecting measurement parameters, including the DC potentials at the input of the inverter and a current in the branch of the bias voltage generation device, and a control device, which is adapted to operate on the basis of the measurement parameters to control the device to detect possible fault conditions and to respond to it. Preferably, the control device is also set up to variably set the value of the bias voltage applied by the bias voltage generation device on the basis of the detected measurement parameters as required. In particular, it may be able to suitably adjust the magnitude of the bias voltage to the voltage of the photovoltaic generator, which varies depending on the time of day, irradiation conditions, temperature and other weather and environmental conditions. For example, for idle operation, a higher bias potential corresponding to the higher generator voltage at idle may be provided, while in operation, the bias potential may be appropriately reduced to avoid high isolation loads and losses associated with a fixed bias value.

It goes without saying that the control device for controlling the inverter on the one hand and the bias voltage generating device on the other hand can be formed at will either by a common integral control unit or by different, distributed control units and executed in software and / or hardware.

The control device can have logic for detecting ground faults or insulation faults on the basis of the measurement parameters detected by the sensor device. In a particularly preferred embodiment, this logic is further configured to detect a creeping isolation fault as follows: First, the logic provides a first value for the bias potential of the bias generator, and the magnitude of the DC potentials at the input of the inverter and the current in the branch of the inverter Bias voltage generator measured. Subsequently, the logic actively modifies the potential of the bias voltage of the bias generator and provides a second value for it. The sensor device then detects the magnitude of the DC potentials at the input of the inverter and the current in the branch of the bias voltage generator for the second predetermined bias potential value. From the measurement and default values for the voltage potentials and currents, the logic then determines the isolation resistances at the positive and negative DC input terminals of the inverter. By comparing the determined insulation resistances with reference values, the Logic recognize the onset of an insulation fault in good time. Advantageously, this recognition of a creeping insulation fault can also take place during operation, for example on a periodic basis.

According to a further aspect of the invention, an additional module for a device for converting an input-side applied electrical DC voltage from a photovoltaic generator is provided in an output-side AC voltage, wherein the device comprises at least one transformerless inverter with a DC input for connection of a photovoltaic generator and an AC output. The add-on module according to the invention has a connection for connection to the AC output of the inverter, biasing means for applying a bias potential to the connection, thereby also affecting the voltage potential at the DC input of the inverter during operation when the connection of the additional module is connected to the AC output of the inverter , and an inductive RF decoupling device, which is configured in operation for RF decoupling of the AC side from the DC side of the inverter when the terminal of the additional module is connected to the AC output of the inverter.

The additional module can thus be retrofitted as a retrofittable unit in an existing photovoltaic system. It can in principle be connected at any point between the inverter output and a mains transformer, housed in the housing of the inverter or the mains transformer or integrated into the inverter or mains transformer. Incidentally, the additional module, in particular its RF decoupling device, bias voltage generation device and control device, can be further developed in the manner described above in connection with the conversion device according to the invention. To avoid repetition, reference is made to the above description of possible embodiments and their advantages.

The invention makes it possible to operate transformerless inverters on photovoltaic generators made of thin-film modules or crystalline modules with back cell contacting, avoiding damage and efficiency reductions due to TCO corrosion and polarization effects and avoiding high leakage currents to the photovoltaic generator causing damage can cause the components of the photovoltaic system.

Further details of advantageous embodiments of the invention are the subject of the drawing, the description or the dependent claims. In the drawings, embodiments of the invention are illustrated. Show it:

1 a photovoltaic system according to the invention for converting a DC voltage of a photovoltaic generator into an AC voltage for feeding into a network with a biasing device and an RF decoupling device according to a first embodiment, in a highly schematic representation;

2 an embodiment of an inverter for use in the photovoltaic system according to 1 in simplified representation;

3 a photovoltaic system with a further embodiment of an RF decoupling device according to the invention, in a highly schematic representation; and

4 a block diagram of a method according to the invention for the detection of insulation errors.

1 shows a schematic diagram of a photovoltaic system 1 , which forms a device according to the invention for converting an input-side applied electrical DC voltage from a photovoltaic generator into an output-side AC voltage. The photovoltaic system 1 has a photovoltaic generator 2 and a three-phase inverter 3 on. The photovoltaic generator 2 has one or more PV modules, not shown in detail herein, which may be formed by any crystalline modules or thin film modules known in the art and connected in series to form a single DC output voltage at the output terminals 4 . 6 of the photovoltaic generator 2 to create.

The inverter 3 is for conversion by the photovoltaic generator 2 at his entrance 7 provided DC voltage in a three-phase AC voltage at its output 8th set up. For this purpose, the inverter has the input 7 with a positive and a negative input connection 9 . 11 on, each with the positive or negative pole 4 . 6 of the photovoltaic generator 2 connected, and the four-pole output 8th to which the three output terminals (L1, L2, L3) 12 . 13 . 14 representing the individual phases of the output AC voltage of the inverter 3 lead, and a neutral output terminal (N) 16 of the inverter 3 belong.

A possible embodiment of the inverter 3 is in a very schematic way in 2 illustrated. The inverter 3 has a DC voltage intermediate circuit 17 with two energy buffers in series 18 . 19 in the form of capacitors C, having one end respectively at the positive and negative input terminals 9 . 11 of the inverter 3 connected and with its other end at a connection point 21 are joined together by one through the inverter 3 passing line 22 with the neutral output connection 16 electrically connected. The DC link capacitors 17 . 18 and the neutral line 22 form part of a balancing circuit 23 , which serves the potentials at the input terminals 9 . 11 of the inverter 3 in terms of the potential at the neutral line 22 set symmetrically.

The inverter 3 also has a switch arrangement 24 on, connected to the input terminals 9 . 11 parallel to the DC link capacitors 18 . 19 connected. The switch arrangement 24 is by a parallel connection of three substantially identical half-bridges 26 formed, each two in series interconnected switch units 27 . 28 have, which are switchable with high frequencies of up to 100 kHz. Although the switch units 27 . 28 shown here only symbolically, preferably low-loss IGBT (Insulated Gate Bipolar Transistor) or MOS field effect transistor switches are used. Parallel to each switch unit 27 . 28 is conventionally connected in each case one (not illustrated) freewheeling diode in the opposite forward direction. The connection point 29 between the switch units 27 . 28 every half bridge 26 is via a connection line 31 holding a storage choke 32 contains, to the respective AC output terminal 12 . 13 respectively. 14 led out. The switch units 27 . 28 the respective half bridges 26 are clocked such that at the output terminals 12 . 13 . 14 a three-phase alternating current for feeding into a network is generated, which preferably has three substantially equal in magnitude, but in each case by 120 ° phase-shifted output currents.

Referring again to 1 is the exit 8th of the inverter 3 with a network 32 , For example, a public power grid or a high voltage transmission network, via an output-side transformer 33 connected for galvanic isolation between the inverter output 8th and the network 32 and for matching the magnitude of the output voltages and currents supplied by the inverter to those of the grid 32 required sizes.

The transformer 33 is here conventionally referred to as a three-phase transformer with a primary side 34 and a secondary side 36 formed, each having three switched to a star primary windings 37 on the primary side 34 and three secondary windings connected to a star 38 on the secondary side 36 exhibit. The central connection point or neutral point of the primary windings 37 forms the neutral connection 39 of the transformer 33 , As illustrated, the neutral connection is 39 via a neutral conductor 41 with the neutral output connection 16 of the inverter and via the inverter-internal neutral line 22 further with the connection point 21 the DC link capacitors 17 . 18 connected.

As further out 1 is apparent, is the neutral connection 39 of the transformer 33 also with a first connection 40 a branch 42 in which a bias voltage generator 43 is arranged, with another connection to the earth 44 connected. The bias generator 43 serves to operate at the port 40 or the neutral connection 39 of the transformer and thus the neutral output terminal 16 of the inverter 3 apply a defined bias potential, which then via the neutral 41 . 22 to the connection point 21 of the DC intermediate circuit 17 is coupled. For this purpose, the bias voltage generating device 43 a constant voltage source 46 on which a constant voltage with variably adjustable size at its with the neutral connection 39 connected output terminal supplies.

For adjusting the bias voltage of the bias voltage generator 43 and for controlling the operation of the inverter 3 Depending on current operating and environmental conditions, a control device is provided, which here only schematically in the form of a block 47 is shown. The control device 47 is with a sensor device 48 coupled, the different measurement parameters on the photovoltaic system 1 recorded and this characteristic values to the controller 47 supplies. The measured measurement parameters include, among others, the DC potentials U _{DC +} , U _DC- at the positive and negative input connection 9 . 11 of the inverter 3 , as in 1 indicated, as well as, for example, the alternating voltages and / or alternating currents at the output terminals 12 . 13 . 14 (not shown here).

The photovoltaic system described so far is known per se and works as follows: The photovoltaic generator 2 converts the radiation energy received from the sun into electrical energy and thereby generates a DC voltage at its poles 4 . 6 , The from the photovoltaic generator 2 supplied voltage depends on the irradiation conditions, the temperature, humidity in the environment of the photovoltaic generator as well as others Factors and is generally in operation in about 600 to 1000 volts. The inverter 3 converts the DC voltage at its input 7 in a three-phase AC voltage at its output 8th from which the three-phase AC voltage across the transformer 33 in the net 32 is fed. The inverter 3 is from the controller 47 according to the through the sensor device 48 measured operating values suitably to provide at its output suitable for feeding AC voltages and AC currents. The transformer 33 adapts these according to the requirements of the network.

During operation, the bias voltage generator applies a bias to the neutral terminal 39 of the transformer 33 and thereby causes a shift of the DC potential at the primary side 34 of the transformer 33 and thus also at the exit 8th , in particular the neutral output connection 16 of the inverter 3 opposite the earth potential around the bias voltage. As a result of the balancing circuit 23 , including the neutral conductor 22 . 41 , are the potentials at the positive and negative input terminal 9 . 11 in terms of bias symmetrical to each other, so be approximately 0 volts at one of the terminals 9 . 11 and about twice the bias voltage at the other terminal 9 . 11 , By selecting an appropriate magnitude and polarity for the bias of the bias generator 43 compared to the ground potential can be ensured in this way that the potentials against the earth for all photovoltaic modules of the photovoltaic generator z. B. are either non-negative or non-positive and are always close to the ground potential.

As is known, photovoltaic modules form electrically chargeable surfaces, which face a grounded frame and thus form parasitic capacitances that can store charge. The parasitic capacitances of the photovoltaic modules are relatively large and are in the range of about 1 μF per cable peak voltage. They depend on design factors, such as the materials used and the charge storage area, and may be due to weather-related factors, e.g. B. when wetted with water, be significantly increased. In any case, the parasitic capacitances of photovoltaic modules are considerably larger than those on the AC side of a transformerless inverter. In 1 are the parasitic capacities of the photovoltaic generator 2 through two capacities 49 . 51 indicated that between the positive and negative pole 4 . 6 of the photovoltaic generator 2 and ground are connected.

In a conventional photovoltaic system with a three-phase inverter, in particular the half-bridge configuration, as in 2 illustrated without the bias generator 43 the AC potential jumps at the neutral connection or star point 39 of the transformer 33 conditioned by the clocking of the inverter. The AC potential jumps with an amplitude which depends on the clock pattern and modulation method used, and with a frequency which is approximately three times the clock frequency of the inverter 3 equivalent. But if the bias generator 43 at the neutral connection 39 is connected, thereby the potential is held on this. The required potential jumps are now via the balancing circuit 23 to the DC input side of the inverter 3 and on to the photovoltaic generator 2 transfer. Due to the voltage jumps, the high parasitic capacitances of the photovoltaic generator must 2 be constantly reloaded, which has high displacement currents at the photovoltaic generator result, which are proportional to the parasitic capacitances and the voltage amplitude. The displacement currents produce leakage currents that drain against earth and that circulate through the system, in particular through the inverter 3 and the constant voltage source 46 the bias generator 43 flow and damage them. In addition, such capacitive leakage currents can be the fault current monitoring of an inverter 3 and thus trigger its disconnection from the network.

To avoid this is according to the present invention, an inductive RF decoupling device 52 for RF decoupling of the AC side from the DC side of the inverter 3 intended. How out 1 can be seen, is the inductive RF decoupling device 52 in the preferred embodiment 1 through an inductance 53 formed between the terminal 40 or the neutral connection 39 of the transformer 33 and the bias generator 43 connected. The inductance 53 has a suitable inductance value in the sense of a high RF resistance in order to sufficiently block or attenuate the high-frequency leakage currents to be expected during operation. The inductance 53 allows the AC potential jumps at the neutral terminal 39 but keeps its potential at a constant mean, that of the bias voltage generator 43 predetermined bias corresponds. This leaves the photovoltaic generator 2 insofar as "quiet", as it comes to no significant voltage jumps and displacement currents at this. By the RF decoupling device according to the invention 52 can otherwise by clamping the neutral point 39 caused capacitive leakage currents at the photovoltaic generator 2 effectively avoided.

3 shows a further embodiment of a photovoltaic system according to the invention 1 with a modified embodiment of an inductive RF decoupling device according to the invention 52 for decoupling the AC output side of the inverter 3 from its DC input side. As far as conformity in construction and / or function is made on the basis of the same reference numerals to the above description in connection with 1 and 2 directed.

The embodiment of the device 1 for converting an input side applied electrical DC voltage from a photovoltaic generator 2 in an output-side AC voltage according to 3 differs from the one after 1 essentially only by the arrangement and design of the RF decoupling device 52 , This is not here between the neutral connection 39 of the transformer 33 and the bias generator 43 but between this and the input terminals of the transformer 33 connected. The from the RF decoupling device 52 and the bias generator 43 formed unit thus has three connections 40a . 40b respectively. 40c for connection to respective output terminals 12 . 13 respectively. 14 of the inverter 3 on. The RF decoupling device 52 has three inductors 54 . 56 and 57 on, each over the connectors 40a . 40b . 40c with one of the AC output terminals 12 . 13 . 14 of the inverter 3 and on the other hand with a common connection point 58 connected, which forms a virtual star point. This embodiment can thus also be applied when the output-side transformer 33 not, as shown here, in a star connection, but is implemented in a triangular circuit.

Incidentally, the individual inductors 54 . 56 . 57 in the embodiment according to 3 in the same way as the inductance 53 to 1 for inductive RF decoupling of the AC output 8th from the DC input 7 of the inverter 3 effective by keeping at constant average of the potential at their terminals 40a . 40b . 40c and thus the inverter output terminals 12 . 13 . 14 in accordance with the bias of the bias generator 43 Potential jumps allow it and high-frequency leakage currents to the DC side of the inverter 3 block or attenuate to leakage currents to the photovoltaic generator 2 to minimize.

In 3 is an additional development of the device according to the invention 1 illustrated. As can be seen, the sensor device detects 48 here the DC potentials at the positive and negative input connection 9 . 11 of the inverter 3 and additionally the current I in the branch 42 the bias generator 43 , By monitoring the current I from the bias generator 43 For example, serious insulation faults that cause ground faults can be detected so that the inverter can consequently be switched off in order to avoid further damage. However, a ground fault can already damage components of the system 1 have occurred.

In the in 3 illustrated embodiment, the control device 47 an additional logic to creeping insulation fault, so insulation error already during their formation, to the photovoltaic generator 2 to recognize. This is done by determining and monitoring the isolation resistances R _{ISO, DC +} and R _{ISO, DC-} at the positive and negative poles, respectively 4 . 6 of the photovoltaic generator 2 accomplished against earth. The insulation resistances R _{ISO, DC +} , R _{ISO, DC-} are in 3 as resistors 59 . 61 , in each case in parallel with the associated parasitic capacitance 49 . 51 of the photovoltaic generator 2 indicated.

The logic according to the invention 62 the control device 47 to detect creeping insulation faults in connection with the flowchart 4 be explained. As in 4 illustrates, the controller gives 47 first, a first bias of the bias voltage generating device 43 before (step S1).

The sensor device 48 then detects the DC potentials U _{DC +} , U _DC- at the positive and negative input terminal 9 . 11 of the inverter 3 and the magnitude of the current I from the bias generator 43 and reports this size to the controller 47 (Step S2).

The process is repeated for a second predetermined voltage. The control device 47 sets a second bias on the bias generator 43 fixed, which differs from the first bias (step S3), and receives from the sensor device 48 Measured values of the DC potentials U _{DC +} , U _DC and the current I in the branch 42 the bias generator 32 (Step S4).

Subsequently or already parallel to the above steps, the controller determines 47 the insulation resistances R _{ISO, DC +} , R _{ISO, DC-} from the measured quantities (step S5). If, for example, the voltages U1, U2 the predetermined bias voltages of the bias voltage generating device 43 are _U1 , U _{DC +, U1} , U _{DC-, U1} or I _U2 , U _{DC +, U2} , U _{DC-, U2,} the measured currents or DC potentials at the first and second predetermined bias voltage can, in a first approximation for the currents I _U1 , I _U2 the following equations are given: I _U1 = U _{DC +, U1} / R _{ISO, DC +} + U _{DC, U1} / R _{ISO, DC} I _U2 = U _{DC +, U2} / R _{ISO, DC +} + U _{DC, U2} / R _{ISO, DC-} .

With the above equations, the two unknown insulation resistances R _{ISO, DC +} and R _{ISO, DC-} can be easily determined.

The insulation resistances determined can, for example, be compared with reference values in order to detect possible insulation faults (S6).

Continuous monitoring of the insulation resistances can advantageously be used to detect creeping insulation faults with the method according to the invention. The method can also be performed quickly in operation by periodically, briefly changing the bias voltage to a different level than the normal operating level.

Numerous modifications are possible within the scope of the invention. Thus, for example, the three-phase inverter 3 be replaced by three single-phase inverters. It is also possible for several inverters to be connected in parallel on the output side. Furthermore, different embodiments for the transformer 33 , z. B. also in the triangle circuit, possible. The RF decoupling device 52 could also have built-up of one or more capacitances and inductors LC filter for blocking certain frequencies in the frequency band in which the high-frequency leakage currents and associated relevant harmonics are expected to have. The embodiments according to the 1 and 3 that alone on the inductors 53 respectively. 54 . 56 . 57 but are preferred because of their simple form of realization and high efficiency. Advantageously, the inventive HF decoupling device 52 (As needed, with the bias voltage generator 43 ) can also be retrofitted effortlessly in existing systems. For this purpose, the RF decoupling device 52 and the bias generator 43 form in a particularly preferred embodiment of the invention, a retrofittable additional module that can be integrated into existing photovoltaic systems.

It is a photovoltaic system 1 for converting a DC voltage from a photovoltaic generator 2 disclosed in an AC voltage, which is a transformerless inverter 3 having high frequency clocked switch units whose DC input 7 at the photovoltaic generator 2 is connected and at the AC output 8th a series circuit of a bias voltage generator 43 and an inductive RF decoupling device 52 connected. The bias generator 43 serves to apply a bias potential to the AC output 8th of the inverter 3 indirectly through the voltage potential at the DC input 7 of the inverter 3 is influenced by. This allows the potential at the photovoltaic generator 2 suitable for the use of thin-film photovoltaic modules or crystalline photovoltaic modules with backside cell contacting. The inductive RF decoupling device 52 is used for RF decoupling of the AC side of the DC side of the inverter 3 to use through the bias generator 43 conditional capacitive leakage currents at the photovoltaic generator 2 to avoid. A combination of the bias voltage generator according to the invention 43 and the inductive RF decoupling device 52 can be realized with little effort and can be retrofitted to existing systems.

Claims (14)

Device for converting an input DC electrical voltage from a photovoltaic generator ( 2 ) in an output-side AC voltage with at least one transformerless inverter ( 3 ), which has a DC input ( 7 ) for connecting a photovoltaic generator ( 2 ) and an AC output ( 8th ) having; with a bias generator ( 43 ) for applying a bias potential at the AC output ( 8th ) of the inverter ( 3 ), whereby the voltage potential at the DC input ( 7 ) of the inverter ( 3 ) being affected; and with an inductive RF decoupling device ( 52 ) for RF decoupling of the AC side of the DC side of the inverter ( 3 ), wherein the inductive RF decoupling device ( 52 ) an inductance ( 53 ) connected in series with the biasing device ( 43 ) in a branch ( 42 ) between earth ( 44 ) and one from the DC side to the AC side of the inverter ( 3 ) passed through neutral conductor ( 22 . 41 ), or the inductive RF decoupling device ( 52 ) one of the number of phase leading output terminals ( 12 . 13 . 14 ) of the inverter ( 3 ) corresponding number of inductances ( 54 . 56 . 57 ), each with an output terminal ( 12 . 13 . 14 ) and on the other hand with a common connection point ( 58 ), wherein the bias voltage generator ( 43 ) between earth ( 44 ) and the common connection point ( 58 ) connected. Device according to claim 1, characterized in that it comprises at least one photovoltaic generator ( 2 ) having at least one photovoltaic module, which is either a thin-film module or a crystalline photovoltaic module with photovoltaic cells contacted on the back side. Device according to claim 1, characterized in that the inverter ( 3 ) in the three-phase configuration with three AC output terminals ( 12 . 13 . 14 ), each associated with one of the three phases of its output voltage, and / or having a half-bridge configuration. Device according to claim 1, characterized in that the inverter ( 3 ) a balancing circuit ( 23 ), which serves the voltage potentials at input terminals ( 9 . 11 ) of the inverter ( 3 ) substantially symmetrically with respect to the bias potential. Device according to Claim 4, characterized in that the balancing circuit ( 23 ) a DC voltage intermediate circuit ( 17 ), the two in series with each other between the input terminals ( 9 . 11 ) of the inverter ( 3 ) connected energy buffers ( 18 . 19 ), and a neutral conductor ( 22 . 41 ) associated with a connection point ( 21 ) of the two energy buffers ( 18 . 19 ) is electrically connected. Device according to claim 1, characterized in that it further comprises an AC output ( 8th ) of the inverter ( 3 ) connected mains transformer ( 33 ), the inverter output voltage to network sizes of a network to be fed ( 32 ). Apparatus according to claim 6, characterized in that the mains transformer ( 33 ) one with the AC output ( 8th ) of the inverter ( 3 ) connected primary page ( 34 ), a secondary page ( 36 ) for connection to a network ( 32 ) and a neutral connection ( 39 ) on the primary side ( 34 ), preferably with one from the DC side to the AC side of the inverter ( 3 ) passed through neutral conductor ( 22 . 41 ) connected is. Apparatus according to claim 6, characterized in that the mains transformer ( 33 ) is a three-phase transformer. Device according to Claim 1, characterized in that the biasing device ( 43 ) a constant voltage source ( 46 ) between earth ( 44 ) and the AC output ( 8th ) of the inverter ( 3 ) connected. Device according to claim 1, characterized in that the neutral conductor ( 22 . 41 ) with a primary-side neutral connection ( 39 ) one at the exit ( 8th ) of the inverter connected mains transformer ( 33 ) connected is. Apparatus according to claim 1, characterized in that a sensor device ( 48 ) for the acquisition of measurement parameters, including the DC potentials (U _{DC +} , U _DC- ) at the input ( 7 ) of the inverter ( 3 ) and a stream (I) in a branch ( 42 ) of the bias generator ( 43 ), and a control device ( 47 ), which is set up on the basis of the acquired measurement parameters, the operation of the device ( 1 ), to recognize possible error conditions and to react to them. Apparatus according to claim 11, characterized in that the control device ( 47 ) is adapted to reduce the size of the voltage generated by the bias voltage generator ( 43 ) set bias voltage based on the detected measurement parameters variable. Apparatus according to claim 11, characterized in that the control device ( 47 ) a logic for the detection of ground faults or insulation faults by monitoring the current (I) in the branch ( 42 ) of the bias generator ( 43 ), including logic for detecting creeping insulation faults on the photovoltaic generator ( 2 ), which is arranged to - actively the potential of the bias voltage of the bias voltage generating device ( 43 ) and to specify the size of the DC potentials (U _{DC +} , U _DC- ) at the positive and negative input terminals ( 9 . 11 ) of the inverter ( 3 ) and the stream (I) in the branch ( 42 ) of the bias generator ( 43 ) for two different predetermined bias potentials (U1, U2), - from the measurement and default values for the voltage potentials (U _{DC +, U1} , U _{DC-, U1} , U _{DC +, U2} , U _{DC-, U2} ) and the currents (I _U1 , I _U2 ) the insulation resistances (R _{DC +} , R _DC- ) at the positive and the negative DC input terminal ( 9 . 11 ) of the inverter ( 3 ) to determine - Compare the insulation resistances (R _{DC +} , R _DC- ) with reference values and use the comparison to detect the onset of an insulation fault. Additional module for a device ( 1 ) for converting an input DC electrical voltage from a photovoltaic generator ( 2 ) in an output side AC voltage, wherein the device ( 1 ) at least one transformerless inverter ( 3 ) with a DC input ( 7 ) for connecting a photovoltaic generator ( 2 ) and an AC output ( 8th ), wherein the additional module comprises: a connection ( 40 ) for connection to the AC output ( 8th ) of the inverter ( 3 ); a bias generator ( 43 ) for applying a bias potential to the terminal ( 40 ), whereby the voltage potential at the DC input ( 7 ) of the inverter ( 3 ) is affected when the connection ( 40 ) with the AC output ( 8th ) of the inverter ( 3 ) connected is; and an inductive RF decoupling device ( 52 ) for RF decoupling of the AC side of the DC side of the inverter ( 3 ) is set up when the connection ( 40 ) with the AC output ( 8th ) of the inverter ( 3 ), the inductive RF decoupling device ( 52 ) an inductance ( 53 ) connected in series with the biasing device ( 43 ) in a branch ( 42 ) between earth ( 44 ) and one from the DC side to the AC side of the inverter ( 3 ) passed through neutral conductor ( 22 . 41 ), or the inductive RF decoupling device ( 52 ) one of the number of phase leading output terminals ( 12 . 13 . 14 ) of the inverter ( 3 ) corresponding number of inductances ( 54 . 56 . 57 ), each with an output terminal ( 12 . 13 . 14 ) and on the other hand with a common connection point ( 58 ), wherein the bias voltage generator ( 43 ) between earth ( 44 ) and the common connection point ( 58 ) connected.

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